Disallow Endless methods, using the Style/EndlessMethod cop

Proposal

Disallow Endless methods, using the Style/EndlessMethod cop.

This proposal was voted on by backend maintainers here.

Implementation Guide

• Configure Style/EndlessMethod cop in gitlab-styles to disallow (more info)
• Refactor existing usages (there aren't many yet, it would be 1 MR). It is used in https://gitlab.com/gitlab-org/gitlab/-/blob/f841a8e37fcee6121c47c8ee3674eb4ef0166622/lib/backup/tasks/uploads.rb#L6 and related classes.